Alkali metal generator



Sept. 23, 1969 mEwo 3,468,806

ALKALI METAL GENERATOR Filed Dec. 6, 1965 FIG-I.

WITNESSES INVENTOR Andreas Nievyold U.S. Cl. 252181.4 1 Claim ABSTRACT OF THE DISCLOSURE An alkali generator comprising the addition of a portion of a gettering material such as tantalum to a mixture of an alkali chromate, aluminum and tugsten or a mlxture of an alkali chromate and silicon to generate copious quantities of the alkali metal without evolving gaseous contaminants.

This invention relates to photosensitive surfaces for electron image devices and more specifically to generator means for producing copious amounts of an alkali metal which reacts to form the photoemissive surfaces.

Photoemissive surfaces are typically employed in a variety of radiation sensitive devices such as television camera tubes, storage tubes, photomultipliers and others. Though the alkali generator of this invention could be utilized in various radiation sensitive devices, the invention will be described below with specific reference to a television camera tube of the image orthicon variety. Such a television camera tube typically includes an image section including a photoemissive surface formed on an end plate of the tube envelope and a thin glass target electrode spaced from the photoemissive surface. Radiation such as visible light is focused as by an optical lens onto the photoemissive surface which in turn generates a photoelectron image whose spatial distribution corresponds to that of the light image. The photoelectron image is accelerated and focused so as to strike the thin glass target electrode with energy sufficient to cause a secondary emission from the glass surface greater than unity. The secondary emission leaves a positive charge pattern upon the glass target surface corresponding to the light image focused upon the photoemissive surface (or layer). The opposite side of the glass target is scanned by an electron beam which approaches the target at a low velocity. Electrons from the scanning electron beam will be drawn to the positive areas of the target surface and will be deposited on the target to neutralize the positive potential pattern established on this member. The beam of electrons discharges the surface of the target and the remainder of the electron beam is deflected back through the envelope of this device and is collected by an electron multiplier to form a video output signal of the television camera tube.

In the type of elevision camera tube described above, the photoemissive surface or layer is formed upon an end plate of the envelope which is transmissive to the radiation which is to be sensed. Typically, a metallic material such as antimony is deposited upon a base plate as by evaporation. Next, an alkali metal is generated and directed upon the layer of antimony. The alkali metal reacts with the antimony layer to provide a photoemissive surface. In a copending application Ser. No. 508,770, entitled Photosurface and Alkali Generators for Producing Same by Clayton D. Spangenberg and assigned to the assignee of this invention, an alkali generator is described which includes a mixture of an alkali chromate with a prescribed portion of boron. As fully set out in this application, the addition of boron results in increased generation of the alkali metal and allows the evolution of the alkali metal to be carried out at significantly reduced temperatures. During the operation of the alkali generator 3,468,806 Patented Sept. 23, 1969 ice as described in the above-identified application and in other generators as well, vapors of the alkali metal are generated by reacting mixture of materials at a high temperature in a metal container. In the generation of the alkali vapor, copious amounts of other gases are released which may act to desensitize or poison the photoemissive layer. In particular, water and oxygen is released from the alkali generator and may react with the evolved alkali metal to form hydroxides or oxides therewith. Further, the metal containers or channels in which the mixture is contained is typically made of nickel. These nickel containers have a tendency to absorb quantities of carbon and when heated tend to form carbon dioxide which is likewise evolved onto the surface of the photoemissive layer. Typically, the carbon dioxide tends to form carbonates with the alkali metal and to poison the photoemissive surface.

In addition to the possible contamination or poisoning of the photoemissive layer, the release of the undesired gases such as oxygen tends to decrease the amount of alkali available to react with the layer of antimony. In particular, the evolved oxygen may react with the alkali metal to form oxides therewith and thereby decrease the amount of alkali available to react with the layer of antimony.

It is thus an object of this invention to provide an improved photoemissive surface or layer for electron image devices.

It is another object of this invention to provide an improved alkali generator capable of generating increased quantities of the alkali metal.

It is a still further object of this invention to provide a new and improved alkali generator which may be operated to generate copious quantities of the alkali metal without evolving other gases which may tend to contaminate the photoemissive surface.

The invention, briefly, achieves the above mentioned and additional objects and advantages through the provision of an improved alkali generator including the combination of an alkali chromate as the source of the alkali metal and a gettering material to absorb or react with those evolved gases which would otherwise contaminate the photoemissive surface. In illustrative embodiments of this invention, a portion of tantalum is added to an alkali generator including the mixture of an alkali chromate, aluminum and tungsten, and to an alkali generator including the mixture of an alkali chromate and silicon. Further, it has been found that the addition of tantalum in an amount in the approximate range of 1% to 10% by weight of the mixture will effectively increase the amount of alkali vapor available to form the photoemissive surface and will also eliminate those gases which would otherwise contaminate or poison the photoemissive surface.

Further objects and advantages of this invention will become apparent in the following specification. For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 is a partial, sectioned view of an electron image device such as image orthicon tube including an alkali generator made in accordance with the teachings of this invention; and

FIG. 2 is a sectional view of the image section of the image orthicon tube shown in figure as taken along lines IIII of FIGURE 1.

Referring now to the drawing and in particular to FIG- URE 1, an electron image device 10 of the image orthicon variety is shown in which the alkali generator of this invention has been incorporated. In particular, the electron image device 10 includes a vacuum sealed envelope 12 comprising an enlarged portion 14 which contains the imaging section of this device, and an extended or neck portion 16 axially aligned with and sealed to the enlarged portion 14. The neck portion 16 is sealed upon the end remote from the enlarged portion 14 with a base member 18 through which a plurality of terminals 20 extend for making electrical connections with external sources of potential. A reading electron gun (not shown) and the electron multiplier section (not shown) are normally incorporated within the neck portion 16. At the other end of the envelope 12, a face or end plate 22 encloses and seals the enlarged portion 14.

Upon the interior surface of the face plate 22, there is deposited a strip 34 of electrically conductive material such as aluminum over a portion of the face plate 22 and extending along a portion of the inner periphery of the enlarged portion 14. A spring bias member 36 is secured as by spot welding to a stem terminal 38 which extends through a bottom segment of the enlarged portion 14. The spring bias member 36 is positioned by the terminal 38 to forcibly abut against the strip 34 and to make an effective electrical contact therewith. Upon the interior surface of the face plate 22 and a portion of the strip 34, there is deposited, as will be explained in greater detail later, a layer 24 of photoemissive material which will respond to a radiation image by generating an electron image whose spatial distribution corresponds to that of the radiation image. Also disposed within the enlarged portion 14 of the envelope 12 is the imaging section which includes a cylindrically shaped accelerating electrode 32 which acts to accelerate the electron emitted by the photoemissive layer 24 onto a storage target 30 which is mounted within a cylindricallly shaped electrode 28. More specifically, the target electrode 30 is mounted upon an annularly shaped support member 33 which has a U-shaped cross section (see FIG. 1). The support member 33 is secured as by spot welding to the interior surface of the cylindrical electrode 28. Further, a cylindrically shaped electrode 26 is disposed remotely on the side of the electrode 28 from the photoemissive layer 24 for decelerating the electrons emitted by the reading electron gun (not shown) which is located within the next portion 16. The electrodes 26, 28 and 32 are supported within the enlarged portion 14 as by a plurality of terminals which extend through the bottom segment of the enlarged portion 14 and which are secured to the various electrodes to make electrical connections therewith. Further, mechanical strength is added to the assembly by the inclusion of a plurality of support rods 60 (only one of which is shown for the sake of clarity) which are made of a suitable insulating material such as aluminum oxide and which are secured to the electrodes 26, 28 and 32 as by a plurality of connecting tabs 62. For a more complete description of the structure and the operation of such an image orthicon tube, reference is made to the US. Patent No. 2,682,479.

The mechanism for the deposition of the photoemissive layer 24 will now be explained in greater detail. As shown in FIGS. 1 and 2, small globules 42 of antimony may be attached to an evaporator filament 40 by melting and applying the antimony to the filament 40 at temperatures below evaporation. The antimony tends to wet the filament 40 and will cling to the filaments as globules 42 to be firmly attached upon cooling. Further, the evaporator filament 40' is supported within the support member 33 at one end as by a tab 56 which is secured to the member 33 and at the other end by a lead 44 which extends through and is supported by the cylindrical electrode 28. The filament 40 makes electrical connection as by the tab 56 to the cylindrical electrode 28 which is in turn electrically connected as by a stem terminal 47 to potential sources external of the envelope 12. The other end of the filament 40 is connected by the lead 44 which is electrically insulated with respect to the support member 33 and the electrode 28 by means well known in the art, and is electrically connected to a stem terminal 46. In this manner, the evaporator filament 42 is mounted within the envelope 12, close to the face plate 22, upon which the photoemissive layer 24 is to be formed. Two or more evaporator filaments may be disposed symmetrically about the axis of the face plate 22; illustratively, a second filament 50 is shown in FIG. 2.

The generator or source of the alkali metal is provided by a plurality of channels 52 made of a suitable electrically conductive material such as nickel and which contains the mixture of substances from which the alkali material is to be generated. The composition, Which will be described in greater detail later, is inserted as by hand within a channel 52. The channel 52 is made from a strip of nickel metal and is rolled so as to form a hollow structure with the edges of the strip secured together by spot welding. It is noted that the seam formed by the spot welding has a plurality of slits therein to allow the escape of the alkali metal. As shown in FIGS. 1 and 2, the channel 52 is mounted upon the cylindrical electrode 28 at one end by a tab '58 which is directly secured by welding to the exterior periphery of the electrode 28. The other end of the channel 52 is secured as by spot welding to the stem terminal 54 which also serves to provide an electrical connection to a point external to the envelope 12. The substances within the channel may be heated by passing a current through the terminal 54, the channel 52, and the stem terminal 47.

The electron image device 10, in which the photoemissive layer 24 is to be formed, is first processed and evacuated, and then baked at a temperature in the approximate range of 375 to 400 C. for one hour to remove the occluded gases from the envelope 12. Then, as is well known in the art, some of the metal electrodes within the envelope are then heated and degassed. After this normal processing of the device, an electric current is passed through the evaporator filament 40 by the use of the stem terminals 46 and 47 to heat the filament 42 to a temperature of approximately 525 to 550 C., which is sufficiently high for evaporating the antimony from the globules 42. In forming the photoemissive layer 24 on the surface of the face plate 22, the evaporator filament 40 is operated to deposit a sufficient amount of antimony to change the light transmission through the face plate 22 to approximately of the normal light transmission through the plate 22 before any antimony is deposited. As is well known in the art, this can be easily determined by projecting a beam of light through the face plate 22 and measuring the intensity of this light beam upon a photocell connected to appropriate measuring circuits. The antimony is allowed to evaporate onto the face plate 22 from the globules 42 until the desired thickness as is indicated upon the photocell has been achieved.

As explained above, the source of the alkali metal is provided from the channels 52 and 53. In accordance with the teachings of this invention, an alkali chromate is mixed with a portion of tantalum to provide an increased yield of the alkali metal and to prevent the poisoning or contamination of the photoemissive layer 24. Typically, the mixture may be obtained by first pulverizing the materials and then mechanically mixing them in a vibrator. The mixed constituents are then pressed into pellets which are again pulverized as by a vibrating mixer. Finally, the materials are placed in the channels 52 and 53 as by hand.

The powdered tantalum, which is to be incorporated within the channels '52 and 53, is readily available in a sufficiently pure form from commercial sources. In the alternative, tantalum may be pulverized to a mesh size of approximately 200 openings per inch which is sufficient to allow powdered tantalum to be easily mixed with the other constituents of the alkali generator. Illustratively, the powder tantalum may be washed in a suitable solvent such as acetone to remove any undesirable traces of grease and then may be dried at a temperature of about C. to remove the acetone. The powdered tantalum may then be added with the other constituents of the alkali generator to thereby remove those undesired gases such as oxygen which would otherwise contaminate the photoemissive layer 24.

In a specific illustrative embodiment of this invention, the alkali generator may be formed of a mixture with portions of aluminum, tungsten, an alkali chromate and tantalum. The alkali chromate may be chosen from the following: potassium chromate, sodium chromate, cesium chromate, lithium chromate, or rubidium chromate. Illustratively, the proportions of a mixture including potassium chromate, tungsten, and aluminum vary between the approximate values of 1:1:1 and 1:1:12. In another illustrative example, the alkali generator may be formed of a mixture of an alkali chromate and silicon. In an illustrative mixture of potassium chromate and silicon, these constituents may be mixed in proportions by weight in the approximate ratios of 1:2 to 1:4. In accordance with the teachings of this invention, an amount of tantalum in the approximate range of 1% to by Weight of the total mixture may be added to achieve an increase of the alkali 'yield and also to prevent the contamination of the photoemissive layer. If the amounts of tantalum is less than 1% by weight of the mixture, there will be an insuf ficient amount of the tantalum to remove or react with the undesired gases. On the other hand, if the amount of tantalum exceeds approximately 10% by weight of the mixture, the excess amount of tantalum will tend to prevent the necessary reaction between the reducing agents such as the silicon or aluminum and the alkali chromate.

After the previously described step of applying a layer of antimony to the face plate 22, a sufiicient electric current is passed through the stem terminals 54 and 47 to heat the channel 52 to a sufficiently high temperature of approximately 900 C. to 1000 C. to cause a chemical reaction in the alkali chromate mixture which will release substantially pure alkali as a vapor in the enlarged portion 14 of the evacuated envelope 12. It is particularly noted that tantalum will remain stable at these high temperatures without evaporating or chemically reacting with the other materials of the alkali mixture. Further, other gettering substances which might have been incorporated into such a mixture could not be subjected to these high temperatures without evaporating to thereby coat the other electrode of the electron image device 10 or without reacting with the other constituents of the mixture to form gases which may attack the photoemissive layer 24. During the evaporation of the alkali metal, the envelope 12 is maintained in the approximate range between 150 C. and 200 C. by

being placed within an oven. This temperature maintains the alkali metal being generated within the tube as a vapor and allows this vapor to pass over and onto the layer of antimony to combine uniformly with the antimony layer. The alkali metal is continued to be deposited upon the face plate until the layer 24 provides a maximum photosensitivity. This may be determined by illuminating the photoemissive layer 24 with a beam of light during the evaporation of the alkali metal. An electric current derived from the photoemissive layer 24 through the stem terminal 38 is measured until it registers a maximum photoemission. When this occurs, the current through the channels 52 and 53 is stopped by turning off the applied voltage and the evolution of the alkali ceases. This method is sufficiently precise so that very little of the alkali metal beyond that which is necessary to properly sensitize the photoemissive surface is introduced into the tube.

Although the present invention has been described with a certain degree of particularity, it should be understood that the present disclosure has been made only by way of example and that numerous changes in the details and the arrangement of parts and elements can be resorted to without departing from the scope and the spirit of the present invention.

I claim as my invention:

1. A source for the generation of alkali metal comprising the mixture of potassium chromate, aluminum, tungsten and tantalum, the ratio of the Weight of potassium chromate, aluminum and tungsten being about 1:1:4, said mixture including a portion of boron in an amount of approximately 1% by weight of said mixture, said tantalum being present in said mixture in an amount in the range of about 1 to 10% by weight of said mixture.

References Cited UNITED STATES PATENTS 2,422,427 6/ 1947 Louden 252181.4 X 2,236,647 4/ 1941 McIlvaine 252-18 1.4 3,096,211 7/1963 Davis 252181.4

TOBIAS E. LEVOW, Primary Examiner J. COOPER, Assistant Examiner US. Cl. X.R. 

